Add ConfigSchema to satisfy schema validation in Serverless 1.79.0

[jeffwasher]

I just upgraded my serverless cli to 1.79.0 and started seeing this warning when deploying:

Serverless: Configuration warning at 'functions.myLambdaFunction.events[0]': unsupported function event
Serverless:  
Serverless: If you prefer to not continue ensure "configValidationMode: error" in your config
Serverless: If errors are influenced by an external plugin, enquiry at plugin repository so schema extensions are added (https://www.serverless.com/framework/docs/providers/aws/guide/plugins#extending-validation-schema)
Serverless: If errors seem invalid, please report at https://github.com/serverless/serverless/issues/new?template=bug_report.md
Serverless: If you find this functionality problematic, you may turn it off with "configValidationMode: off" setting

This PR just defines the schema for custom properties and the preExistingCloudFront function event as per https://www.serverless.com/framework/docs/providers/aws/guide/plugins/#extending-validation-schema.

[horike37]

@jeffwasher
Thanks! This is great. more safety 👍

[horike37]

🎉 This PR is included in version 1.1.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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